We will examine how signals from various Stiles pi mechanisms (threshold mechanisms) interact in several spatial detection tasks. Stiles and we have shown that, with suitable choices of colors and spatial targets (our work), the pi mechanisms can be approximately isolated by chromatic adaptation; the pi mechanisms then have spectral sensitivities resembling the absorption spectrum of the photopigment rhodopsin, but peaking at different wavelengths. These adaptation processes may thus reflect properties of visual receptive mechanisms (perhaps cone photopigments) very early in the visual pathways. When two test patterns or backgrounds of different colors are mixed, the pi mechanisms often show interactions that appear to depend critically on the size or spatial frequency of the stimuli. We will study these interactions using the methods of threshold summation and facilitation and spatial adaptation and masking. Gratings of one or more colors will be generated as interference patterns on the retina with lasers -- this eliminates the problem of chromatic aberration. Our proposed system will produce phase-locked gratings or gratings that drift or flicker in counterphase. Homogeneous backgrounds of filtered tungsten light will be used for chromatic adaptation to isolate pi mechanisms. To investigate spatial adaptation and masking, the visibility of a test grating will be measured as a function of the color and spatial parameters of a second grating that is presented simultaneously or successively with the first grating. We will investigate the following hypotheses suggested by our previous work with gratings: (1) The short wavelength pi mechanisms act independently when the stimulus is coarse (low spatial frequency) but interact strongly with other pi mechanisms at high spatial frequencies -- perhaps additively in detection of luminance contrast. (2) The middle and long wavelength pi mechanisms show color-opponent interactions at low spatial frequencies, but act additively or independently at high spatial frequencies. We will also measure the spatial tuning properties of the neural systems associated with the "blue" mechanisms in spatial adaptation and masking experiments.